Cementitious mixtures comprise mixtures of hydraulic cement and water. Hydraulic cements may be mixed with water and small aggregate to form mortars, a cementitious mixture. Concrete, a cementitious mixture, is a mortar which additionally includes large aggregate. Cementitious mixtures include a wide and increasing variety of components. Such components include, but are not limited to, hydraulic cement, pozzolans, fillers, water, coarse and fine aggregate, and chemical additives and admixtures. Chemical additives, admixtures, and concentrates of chemical admixtures include, but are not limited to, set retarders, set accelerators, air-entraining or air-detraining agents, corrosion inhibitors, any other dispersants for cement, pigment dispersions, wetting agents, water soluble polymers, strength enhancing agents, rheology modifying agents, water repellents, and any other admixture or additive that does not adversely affect the properties of the cementitious mixture. Many chemical additives, admixtures, and concentrates of chemical admixtures are added to a cementitious mixture in liquid form.
As used herein subsequently, the term “additive” will comprise additives, chemical admixtures, and concentrates of chemical admixtures.
Dispensing additive liquids for use with cementitious mixtures requires care in the precision and accuracy of dispensing to ensure that the desired quantity of additive liquid is dispensed for inclusion into a batch of cementitious mixture. Failure to dispense with the requisite precision may result in undesirable variation in cementitious mixture quality and performance between batches. Failure to dispense with the requisite accuracy may result in undesirable cementitious mixture quality of a given batch.
A common method in the industry for addition of additive liquids to cementitious mixtures which achieves acceptable levels of precision and accuracy for determining the quantity of additive liquids is the accumulation and measurement of the additive liquid in the a vessel such as a weigh vessel or a volumetric accumulator. A weigh vessel is a container engaged with a scale by which the vessel or the vessel and contents accumulated therein may be weighed. Scales are well-established technology that permits very high level of precision and accuracy in the determination of the weight of accumulated material. By taking the weight of the vessel with its accumulated contents and subtracting the known weight of the vessel, the weight of the accumulated contents of the vessel may be determined. The weight of the contents may be used to determine the quantity of the contents because, at a constant acceleration, weight is proportional to mass. After weighing the vessel and its contents, the accumulated contents are discharged from the vessel so that they may be incorporated into the cementitious mixture.
A disadvantage of using a weigh vessel is that the simplest way to establish the weight of the additive liquid contents is to accumulate them and allow them to come to rest in the vessel over some repose time prior to weighing the vessel. Failure to allow the accumulated contents to come to rest in the vessel can result in dynamic forces contributing to the measured weight of the contents, and a resultant inaccurate measurement of the quantity of additive liquid. This repose time for the liquid to come to rest adds undesirable time, and therefore cost, to the method of determining the quantity of liquid.
Another disadvantage of using a weigh vessel or any other vessel in which the additive liquid is accumulated and measured is that, due to various physical properties which may include, but are not limited to adhesion, cohesion, viscosity, and apparent viscosity, some of the quantity of an additive liquid weighed in a weigh vessel may remain in the vessel after the weighing operation and discharge of the contents from the vessel. This remaining quantity of additive liquid is part of the additive liquid which was weighed such that, failure to incorporate this remaining quantity of additive liquid into the cementitious mixture may result in unacceptable inaccuracy.
One common way of reducing or eliminating the problem of a remaining quantity of additive liquid in the weigh vessel is by flushing at least a portion of the remaining quantity of additive liquid from the weigh vessel with a purgative fluid. Subsequently, the purgative fluid and the flushed portion of the remaining quantity of additive liquid is incorporated into the cementitious mixture. By so doing, the quantity of liquid remaining in the weigh vessel is reduced to levels which do not introduce unacceptable inaccuracy into the method.
A disadvantage of flushing the remaining quantity of additive liquid from the weigh vessel with a purgative fluid is that, in order to assure precision in the method, it is necessary to flush all or substantially all of the entire volume of the weigh vessel in order to assure that all or at least a substantial fraction of the remaining quantity of additive liquid is flushed therefrom. In order to flush all or substantially all of the entire volume of the weigh vessel, it is necessary to fill or substantially fill the entire volume of the weigh vessel with the purgative fluid. Cementitious mixture batches greatly vary in volume and can be quite large. The volumes of additive liquids for use in large cementitious mixtures vary in volume proportionately to the cementitious mixture batch volume and can also be proportionately large. Accordingly, a weigh vessel for commercial and industrial applications must be of sufficient size to accommodate the maximum volume of liquid to be weighed. Because this maximum volume of additive liquid to be weighed is proportionately large, the amount of purgative fluid required to fill or substantially fill the entire volume of the weigh vessel is also proportionately large. Filling or substantially filling the weigh vessel with purgative fluid can be costly both in time to fill and in the cost of the purgative fluid. An alternative is to maintain a smaller volume weigh vessel and resort to multiple additions of additive liquid. While this allows for a smaller weigh vessel to be employed, it is more time consuming and undesirably increases the time to measure and discharge the desired amount of additive liquid.
Because the volume of purgative fluid may be large and may be incorporated into the cementitious mixture, it is important to use a purgative fluid that will not adversely affect the properties of the mixture at high volumes. Because water is the most common purgative fluid, and because extraneous water in a cementitious mixture has strong adverse affects on the strength and rheological properties of the cementitious product, preventing the addition of extra water in the form of the purgative fluid is important to product quality. It is common to reduce the volume of water added to the cementitious mixture prior to incorporation of the additive or additives by a volume of water equal to the volume of the purgative water needed to flush the weigh vessel. This reduction in volume of water is referred to as “hold-back” water because it is held back from the cementitious mixture. By so doing, upon addition of the purgative water, the amount of water in the cementitious mixture reaches the desired amount.
A drawback of using a method in which the volume of water in the cementitious mixture is regulated by using hold-back water, is that the smallest batch of cementitious mixture which can be produced with the proper quantity of water (“minimum batch volume”) corresponds to one having exactly the volume of the hold-back water. Because the volume of the hold-back water is equal to the volume of the purgative fluid needed to flush the weigh vessel and because the weigh vessel is relatively large for commercial and industrial applications, the minimum batch volume is correspondingly large. This minimum batch volume limits the size of cementitious mixture batches and delays the point at which the cementitious mixture can be mixed, thereby limiting the utility of a given weigh vessel.
The state of the art methods for dispensing liquid additives to cementitious mixtures are slow, costly, and create an undesired lower limit to the cementitious mixture batch volume. Accordingly, a need exists in the art for an improved device and method for dispensing additive liquids for addition to cementitious mixtures.